This invention relates to a process for the de-inking of printed waste paper. In particular, this invention relates to such a process in which waste paper is treated in a pulper at alkaline pH values, with alkali silicate, oxidatively active bleaching agents, fatty acids and/or resinic acids containing more than 10 carbon atoms, and dispersing agents. Such a process also includes the separation of the detached printing ink particles from the fibrous material suspension through flotation and/or washing.
For purposes of the present invention, the term "de-inking" means the removal of printed inks from waste paper fiber suspensions. In the de-inking process cleaned fibers are reused for the manufacture of various qualities of paper, such as printing stock, sanitary papers, writing papers, and the like. It is important that in the course of the de-inking process, the printed waste papers are pulped with an addition of chemicals causing the printing inks to become detached from the fibers. The detached printing inks are subsequently removed from the fibrous suspension through flotation, washing, or a combination of such processes. In both the flotation as well as the washing processes, the waste paper typically is pulped in an alkaline environment with the addition of soda lye, alkali silicate, and hydrogen peroxide, as well as a collector, surfactant, and a sequestering agent. Then, in the flotation cells, the fibrous material is cleaned by floating out the printing inks, or by use of a filter, the fibrous material is washed to remove the printing inks. The fibrous material which is recovered may then be utilized in the production of paper.
The addition of alkali to the waste paper results in a swelling of the cellulose fibers and removes the hydrogen bridge bond, making possible easy decomposition of the waste paper. The alkali also functions to saponify fatty acids, if such materials have been used as collectors. The soaps cause a reduction in surface tension or interfacial tension, and detach the printing inks. In part, the soaps precipitate with the agents which cause the hardening of the water, in the form of insoluble calcium soaps, and attach to the printing inks, making them hydrophobic.
In order to improve the detaching of the printing inks and the dispersion, use typically is also made of nonionic surfactants such as fatty alcohol polyglycolethers, or alkylphenol polyglycol-ethers, which also aid in the development of foam in the flotation cells.
Hydrogen peroxide is the oxidatively active bleaching agent generally used to prevent yellowing of the fibers which may result from the use of the alkali. Other chemicals, such as sodium silicate and sequestering agents, stabilize the bleaching liquor. Thus, the sequestering agents prevent rapid decomposition of the hydrogen peroxide by heavy metal ions, such as copper, iron, manganese, or the like.
In German Pat. No. 2,903,150, there is listed the following combination of chemicals, based upon the weight of the waste paper:
2-5% by weight of alkali silicate (sodium silicate) PA1 0.5-3% by weight of hydrogen peroxide (100%) PA1 0.5-2% by weight of sodium hydroxide (100%) PA1 0.3-2% by weight of fatty acids, or salts PA1 0.02-0.5% by weight of non-ionic dispersing agent
If sequestering agents are used, they may be added in a quantity of up to 0.4%, by weight, as in the form of commercially available material which has a 40% concentration. The stock density in the pulper is from 4 to 7%, by weight, and the temperature is 20.degree. to 50.degree. C. Depending upon the intended use of the paper to be reprocessed, different qualities of waste paper are used, such as newspaper, magazines, NCR papers, labels, and the like. In the event of a later use in sanitary papers, one uses newspapers with a low ash content, whereas for later use in printing papers, one uses a waste paper mixture of newspapers and magazines at a ratio of 1:1. Frequently, the difficulty in removing the printing inks from the waste paper depends upon the printing method and the printing ink composition.
In general, the degree of whiteness of a waste paper mixture consisting of the printless margin of newspapers and magazines in a ratio of 1:1 is 61 to 63%. When 1:1 mixtures of printed newspapers and magazines are de-inked, one generally gets a degree of whiteness of 58 to 60%, when fatty acids are used. In such an instance, the fatty acids are added in the liquid state, immediately before the waste paper is ground up. Unless the fatty acids are already liquid at room temperature, the solid fatty acids require the use of a melting device. Under the mild reaction conditions in the pulper, the fatty acids saponify very slowly, causing coarse flocculation. The calcium soap formed on the surface of the flocks impedes further saponification of the enclosed fatty acids and prevents them from becoming active. The unsaponified fatty acid becomes attached to the fibers and, by making them hydrophobic, results in higher fiber losses during flotation.
An object of the present invention is to improve the known de-inking processes to overcome the aforementioned disadvantages.